Mr Obama has decided to buy a new car. He reckons the old presidential limo, known as The Beast, is getting a bit long in the tooth and that it’s time for something a bit better.

As a result, all three of America’s big three automakers – General Motors, Ford and Fiat – have been invited to pitch their ideas. And I can’t help wondering... why?

Yes, Mrs Queen has a specially made Bentley for official engagements, but other heads of state seem perfectly capable of getting about in normal cars. Mrs Merkel doesn’t feel the need to swan about in a limo. But then, why would she? If you’re running Germany, you can turn up in an S-Class, wallowing in comfort and patriotism at the same time.

It’s much the same story with Mr Cameron. He can choose from the new Range Rover or a Jaguar XJL, and, of course, whoever is running Italy this week can rock up in a Maserati Quattroporte.

Things are a bit more problematic for Monsieur Hollande. Now that none of the French carmakers do anything remotely presidential, he is forced to go around Paris on a small moped. He must be a laughing stock whenever the EU power brokers meet... right up to the point when Romania’s president turns up in his Dacia Sandero.

Oh, come on, there must be some snobbishness about this. All of the world’s press are waiting outside the door of whichever palace has been chosen to host the meeting, and you, the president of a major European country, has to arrive in a cast-off Renault Clio.

But that’s what he does. Gets out, straightens his tie, turns to the Greek chap and says, “You can shut it. You had to come on a rented scooter.”

The thing is, though, that all of these leaders are juicy targets for terrorists. All are guarded by crack security teams. And all are as important to their country as Mr Obama is to his. So why does he feel entitled to have a specially made, one-off, armour-plated Beast? Why can’t he just turn up in a Chevrolet Impala. Or, since he’s so obsessed with battery power, a Tesla?

No, scratch that one. Because, when Mr Obama was campaigning, he made a solemn promise that by the end of 2012 the entire White House fleet would be either plug-ins or hybrids. And so far, the total number of vehicles on the books that match that description is... um... zero.

The fact is, though, that the Americans want to see their man in a vehicle quite unlike anyone else’s. It’s a new-money thing, I guess. But, that said, in these hard times, they will not want him to sign a blank cheque. Which means the new limo may have to be built with one eye on the bank balance.

The current car isn’t really a car at all. It’s built on the platform of a Cadillac truck and powered by a truck engine as well. So that it doesn’t look like the sort of thing farmers and murderers use, it has various bits of stock Cadillac trim pieces, such as the headlights from an Escalade. But there are several features which are not stock at all. The blood bank in the boot, for instance, which is filled with a supply that matches the president’s. And the night-vision system. And the door seals which are designed to resist a chemical attack.

Then we get to the armour plating. Except we don’t, because it’s classified. We are told, however, that it can shrug off a direct hit from anything which a terrorist might reasonably be able to carry.

All of this comes with a few downsides. Size is one. Watching The Beast park in Downing Street is hysterical. And weight is another. It beached itself in Ireland a few years ago. But the greatest problem, in these times, is cost.

And that’s going to be the big issue for the big three carmakers as they struggle to design a replacement. How do you come up with something that reflects America’s position in the world and which can take a direct hit from an RPG? That doesn’t cost eleven hundred and twenty-thirteen billion dollars?

Needless to say, I have a few ideas of my own. First of all, the car only needs space for a driver and the president. As we know, the rest of the security detail has to run alongside. Which means it needn’t be capable of doing more than 12mph, or they’ll get left behind.

Of course, it must be airtight and, of course, it must be armour-plated. It should also come with an inbuilt weapons system, and, on top of all this, it’s got to look cool and impressive when pulling up outside the palace of a foreign dignitary.

Well, I’m sorry, but America already makes a vehicle which meets all of those requirements: the M1A1 Abrams main battle tank.

It’s thought that 6,000 have been produced over the years, and, if we are honest, today they are useless. They can’t be used in Afghanistan, because you won’t win the battle for hearts and minds if you turn up in a 60-tonne tank. And they won’t be used against Russia because... well, they just won’t, because these days that’d be silly.

Which means they are all just sitting about doing nothing. And that’s madness, because they really are very superb. On smooth ground, they can travel way faster than the chaps running alongside – up to 60mph in fact, because they are powered by gas-turbine motors.

And they come with the best armour in the world. It’s British. But unlike the current limo, they aren’t designed to simply take fire. They can also shoot back.

I once watched an Abrams, travelling at 30mph over rough ground, hit a truck parked a mile away. Watching that barrel lock onto the target and just stay there, irrespective of what the tank was doing, is one of the most impressive things I’ve ever seen.

Inside? Well, they are not luxurious. But they are easy to drive. In fact, if you go on the internet, there’s some footage of me racing AA Gill from The Sunday Times through the streets of Baghdad in two Abrams. And if he can manage, a Secret Service numpty should have no problem at all.

Parking is easy too. Partly because they can turn round in their own length, but also because they can sort of go over things that are in their way. Certainly, there will be no more embarrassing incidents of the president becoming beached.

You know I’m making sense. Obama gets to run around in an all-American – apart from the armour – product. It costs nothing because it’s sitting around anyway. It will impress, especially if it is painted black. And it’s better at protecting him than a converted Cadillac pickup truck.

But, of course, the Americans will not take up my suggestion. Which brings me on to the fields of Challenger 2s that are currently sitting around on various British army bases gathering dust. Hello. Mr Cameron. Hello.